This invention relates to an apparatus and method for forming an envelope from a sheet of a relatively brittle material and, more particularly, to such an apparatus and method for forming such an envelope for use as a battery separator.
Traditionally, wet cell batteries employ at least one pair of opposite polarity planar electrodes, normally referred to as plates, which are closely spaced in a parallel relationship and immersed in a liquid electrolyte to form an electrochemical couple. A great majority of these type batteries employ a plurality of the opposite polarity plates with the plates being normally arranged in an upstanding face-to-face relation, and with the exact number of pairs in a particular battery depending upon the capacity and/or voltage desired.
To avoid adjacent opposite polarity plates from coming into direct physical contact and thereby causing arcing and/or short circuiting, it is necessary to electrically insulate the adjacent plates from each other. To achieve this insulation, electrically insulative material, usually in the form of sheets similar in size to the electrode plates, and commonly referred to as separators, have been positioned between the opposite polarity plates, with the insulative material being permeable or semipermeable to the liquid electrolyte to sustain the ionic conduction required for the battery action. In older designs, the separators were formed of rubber, wood, and glass while more recent designs have used plastic materials.
Although these types of separators were adequate in many applications, some deficiencies in their insulating capability did develop, especially in connection with relatively heavy-duty industrial-type batteries such as traction batteries, etc. In particular, it has been discovered that, by reason of the close proximity of the exposed side edges of the adjacent plates, dendrites form on and project out from the edge portion of one or the other of the plates, especially after the battery had been in use for a relatively long period of time. These dendrites cause short-circuiting and/or arcing between the adjacent plates which, of course, reduces the capacity and/or output voltage of the battery.
In order to overcome this problem, recent battery designs have included an open ended envelope-type separator of an insulating material which covers the side edge portions of the plates and prevents the above-mentioned short circuiting and arcing.
In a parallel sense, the state of the art has also developed with respect to the materials used for the separators. In particular, recent designs have formed separators of a microporous plastic material, including polyethylene, vinyl resin, and the like, which has excellent insulating capability yet is relatively low in weight and cost. An example of this type of separator is disclosed in U.S. Pat. No. 3,696,061, issued Oct. 3, 1972 in the name of Selsor et al., the disclosure of which is hereby incorporated by reference. The Selsor et al. patent is assigned to the same assignee as the present application. And although battery separators of the type disclosed in the aforementioned Selsor et al. patent have found widespread commercial acceptance, these separators suffer from the disadvantages of being relatively brittle and consequently, heretofore have been incapable of being folded into the desired envelope configuration on a high production, relatively inexpensive basis.